Thermoformable products such as acrylic spas, shower stalls, burial caskets, and the like inherently suffer from the same drawback. Economic and manufacturing limitations make the finished product susceptible to fracturing at stress bearing points. For instance, the steps and bottom of spas, shower stall floors, and any point of curvature of an acrylic sheet are all prone to cracking. These stress points develop because the acrylic sheet flexes under load and the stresses concentrate in specific areas to a level high enough to cause the part to fail.
Thermoformable sheets are typically heated above their heat distortion temperature (HDT) in the initial stages of a thermoforming process. This causes the sheet to expand proportional to the material's coefficient of thermal expansion (COTE). After the forming stage of the process, the sheet is allowed to cool. The cooling stage causes the sheet to shrink unless the shrinkage is inhibited by a foreign object such as a typical prior art mold or insert. As the sheet cools around a rigid insert, it cannot shrink easily, if at all. Therefore, stress accumulates in some areas, such as in comers. Thinning of the material also occurs in these areas. Usually, the inserts will restrict the contraction of the sheet in areas of curvature and the material becomes susceptible to fracturing. The thermoformed product may even crack during cooling to room temperature. This deficiency is characteristic of all plastic sheet materials and is particularly evident with acrylic sheets, such as poly(methyl) methacrylate (PMMA). It may also occur in composite sheets made of PMMA and a thermoplastic substrate layer such as acryonitrile butadiene styrene (ABS), or other materials.
Attempts have been made to reinforce acrylic and similar type products. Favaron, in U.S. Pat. No. 5,400,556 discloses a step system for swimming pools comprising a unitary stair module including a stair shell formed of plastic sheet material and a plurality of elongated rigidizing members which are encapsulated within the plastic sheet material of the shell, and support braces for supporting the unitary stair module.
While the above reference discloses a method for providing support to a thermoplastic sheet material, the difficulties associated with stress build up in and near areas of curvature have not been avoided. These difficulties are not as critical in products such as a modular step systems wherein there are few areas of curvature greater than 90.degree.. That is, where the thermoplastic material has been deformed by more than 90.degree.. Additionally, the above reference does not address the difficulties encountered with sheet material thermoformed over an area greater than a few square feet.
The present invention alleviates the stress associated with deformation of an acrylic or other thermoplastic sheet by more than about a 90.degree. angle. The present invention also alleviates the stress associated with thermoforming large areas of sheet material over male type mold configurations. By male type configurations, we mean thermoformed sheets placed over a shaped profile, wherein the shape raises from the horizontal plane and the thermoforming process allows the sheet to be draped over it. Inserts are positioned on top of the male type configuration or the horizontal surface. The inserts will be enveloped by the formed sheet, so that they will be trapped by it. In the present invention, "removable plug" section(s) are positioned within the inserts, and can be removed when the thermoformed part is removed from the mold. In doing so, the inserts are allowed to become shorter, thus the thermoformed material is not subjected to the same level of stress, that a thermoformed material would be exposed to with a typical molded insert. This technique allows for a stronger, more resilient finished product that is not as susceptible to fracturing.